Integrated circuits (ICs) historically were manufactured using a 5-volt process, wherein a voltage level of 5 volts indicated a "high," and 0 volts indicated a "low." As integrated circuits have become faster, and smaller transistor geometries have been implemented, powering the integrated circuits at lower voltages is required to prevent long term reliability degradation due to excessive voltage across the transistors' gate oxide.
Microprocessors, such as the INTEL Pentium.RTM. Processor, are examples of integrated circuits that have progressively been manufactured using lower voltage processes. Higher clock rates and correspondingly higher performance are made possible by smaller transistor geometries. For example, Pentium processors are available which operate at 5V, 3.3V, and 2.9V. The inputs and outputs of the microprocessor (I/O buffers) are referenced to the power supply, thus a 5V microprocessor typically drives and receives 5V levels, while a 3.3V microprocessor typically drives and receives 3.3V levels, and a 2.9V or lower power supply microprocessor typically drives and receives 2.9V levels.
Additionally, microprocessors may be developed which operate at lower voltages than 2.9V.
It is sometimes desirable to upgrade a microprocessor with a more recent one. A problem results if the upgrade microprocessor is manufactured with a lower voltage process than that of the original. This is because the inputs and outputs of the upgrade microprocessor may not be compatible with the voltage level of the circuit board into which the microprocessor is being replaced.
For example, if a motherboard were designed for a 3.3 V microprocessor, and the motherboard is to be upgraded at some later time to a 2.5 V (or lower) upgrade microprocessor, a problem arises because the upgrade microprocessor cannot be swapped directly. If the 3.3 Volt signals on the motherboard were directly input to the 2.5 V upgrade microprocessor, this could cause long term reliability problems for the 2.5 V upgrade microprocessor.
Some inputs and outputs of the upgrade microprocessor may by design have buffers that are tolerant of voltages higher than the power supply voltage of the microprocessor. For example, an Intel Pentium microprocessor may operate at 3.3 V but be tolerant of 5.0 V signals. Some I/O buffers of the microprocessor, however, do not have a tolerance for high voltage signals supplied by the motherboard. To maintain compatibility, a way of providing a lower input voltage to the I/O buffers is needed.
One method to provide the upgrade microprocessor compatibility with the motherboard is by redesigning the I/O buffers of the upgrade microprocessor so that they are tolerant of higher voltages. This, however, adds more complexity and expense to the cost of making the upgrade microprocessor.
An alternative method to provide the upgrade microprocessor compatibility is by redesigning the motherboard with a voltage level shifter. The voltage level shifter is positioned between signals on the motherboard and the inputs and outputs of the upgrade microprocessor. Various voltage level shifters, also called voltage translators or bus switches, are available on the market. For example, Quality Semiconductor sells voltage level shifters QS3384 and QS3L384, and National Semiconductor sells the 74LVX3L384. Placing the voltage level shifter on the motherboard decreases the amount of real estate available. Also, this is not possible for a processor installed at a later date.
FIG. 1 shows a configuration for a voltage level shifter 10 coupled between two devices 12 and 14. The first device 12 operates at a higher voltage range than the second device 14. For example, the first device 12 can be a chipset that generates signals in a voltage range of 0 to 5.0 V. The second device 14 can be a microprocessor operating in a voltage range of 0 to 3.3 V. The first device is coupled to the voltage level shifter via a signal line 20. The signal line 20 is coupled via the voltage level shifter to signal line 22, which is coupled to an input of the second device 14. The first device 12 is also coupled to the voltage level shifter 10 via a signal line 30. The signal line 30 is coupled via the voltage level shifter 10 to signal line 32, which is coupled to an input/output (I/O) or an output of the second device 14. A pull-up resistor 40 couples the signal line 30 to a voltage level of the first device 12, i.e., 5.0 V level.
The voltage level shifter 10 is able to shift voltages bidirectionally. With regard to signals coming from the first device 12, the voltage level shifter 10 lowers the voltage level of a signal operating in a first voltage range to provide a signal operating in a second voltage range. With regard to signals coming from the second device, the voltage level shifter provides an output signal operating in the first voltage range of the first device. The voltage level shifter 10 uses a pull-up resistor to provide an output signal in the first voltage range, wherein an open-drain or open-collector output of the second device allows the pull-up resistor to raise the voltage level. In either case, the output acts as an open circuit for a "high" level, and the output is connected to ground for a "low" level. Referring back to FIG. 1, when the second device 14 has its output in the "open" state driven to the voltage level shifter 10 on signal line 32, the signal line 30 is pulled up to the voltage level of the first device because of the pull-up resistor. When the second device 14 grounds its output to the voltage level shifter 10 on signal line 32, the voltage level shifter 10 provides a grounded output on signal line 30.
Thus, for decreasing the voltage level, the voltage shifter receives a signal at the higher voltage range and provides a signal at the lower voltage range. For increasing the voltage level, however, the voltage shifter receives an input which is open. In response to the open input, the voltage level shifter provides a signal at the higher voltage range by way of a pull-up resistor.
A voltage level shifter can be set up to level shift voltages to a particular voltage level based upon a voltage level input and based upon the voltage level to which the pull-up resistor is coupled.